POWDERY MILDEW (PM) NEWS AND REMINDERS. Your (almost) annual brief review of PM biology with respect to management considerations.

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1 GRAPE DISEASE CONTROL, 2015 Wayne F. Wilcox, Department of Plant Pathology, Cornell University, NY State Agricultural Experiment Station, Geneva NY POWDERY MILDEW (PM) NEWS AND REMINDERS Your (almost) annual brief review of PM biology with respect to management considerations. (i) The fungus overwinters as minute fruiting bodies (chasmothecia, which used to be called cleistotheia) that form on leaves and clusters during late summer and autumn, then wash onto the bark of the trunk where they survive the winter. Spores are produced within them, and in New York, most such spores of any consequence are discharged between bud break and bloom (more or less) to initiate the disease, after which it can spread rapidly via the millions of new spores produced from each of these "primary" infections. Thus, the amount of fungus capable of starting disease this year is directly proportional to the amount of disease that developed last year. An important consequence of this is that disease pressure will be higher, and PM sprays during the first few weeks of shoot growth are likely to be far more important, in blocks where PM control lapsed last year as compared to blocks that remained clean into September. (In much of the Northeast, chasmothecia initiating from infections that occur after Labor Day are unlikely to mature before temperatures become limiting and/or frost kills the leaves and eliminates their food source.) The annual illustration of what this means: Some years ago, we conducted an experiment in a Chardonnay vineyard where we either (a) sprayed up through Labor Day, maintaining a clean canopy the entire season; (b) quit spraying other vines a month earlier, to represent a planting with moderate levels of foliar PM by the end of the season; or (c) quit spraying in early July, to represent a planting where PM control broke down for one reason or another. The next spring, the levels of chasmothecia (number per kilogram of bark) in these treatments were (a) 1,300; (b) 5,300; and (c) 28,700, respectively. Now, consider a hypothetical case where 20% of the overwintering spore load is discharged during the first couple of weeks after bud break (a reasonable scenario, based on published studies). But 20% of what? In the clean treatment (a), this number might be relatively inconsequential; in dirtier treatment (b), it's equal to the entire seasonal supply on the clean vines; and in treatment (c), it's four to five times greater than the entire seasonal supply on the clean vines. Not surprisingly, this makes a difference that is, the degree of control provided in one season can affect the success of the control program (or its required intensity) the following year. When we intentionally waited until the immediate prebloom period to apply a minimal spray program to the same vines the year after allowing our variable foliar disease levels to develop, the resulting cluster disease severities were (a) 11%, (b) 22%, and (c) 48% (proportion of the cluster area infected) in these respective groups, even though all vines were sprayed exactly the same during the second season. WHY: Higher disease in Year 1 = More inoculum to cause primary infections early in Year 2 = Many more new ("secondary") spores produced from these early infections by the time fruit were formed and highly susceptible to PM. Meaning that the higher the spore level was when berries 1

2 were susceptible, the less effectively the fungicide program controlled it. This is a basic concept that we re all aware of, but here s a concrete example of just how important it is to reduce inoculum levels in a vineyard as part of a disease management program. Depending on the disease involved, sometimes this can be accomplished in part by sanitation procedures that remove diseased plant organs before the season begins (e.g., cankered wood, black rot mummies). And in annual production systems, it can be greatly facilitated by crop rotation. But in a perennial crop like grapes, the very best way to minimize inoculum levels at the start of one season is by minimizing disease development the previous year, by implementing good control programs. This is a major reason that some blocks are almost always clean and some are almost always otherwise, i.e., it s a virtuous or vicious circle. (ii) Powdery mildew functions as a compound interest type of disease, that is, a few infections can snowball and build up to many in a short period of time if conditions are favorable for reproduction of the fungus (a high interest rate ). The most important factor that governs the rate of reproduction is temperature, with a new generation produced every 5 to 7 days at constant temps between the mid-60's and mid-80's ( F); more details are provided in the NY and PA Pest Management Guidelines for Grapes and in an on-line fact sheet. Thus, days in the 80's and nights in the 60's and 70's provide ideal conditions for the fungus 24 hr a day. Conversely, a very cold night or two can seriously set the fungus back while it s trying to ramp up in the spring, as discussed a little farther below. (iii) High humidity also increases disease severity, with optimum conditions for development about 85% RH. Although there is no practical threshold level necessary for the disease--pm develops to some extent over the entire range of humidities that we experience during the growing season--research has shown that disease severity is twice as great at a relative humidity of 80% versus an RH of 40%. Vineyard sites (and canopies) subject to poor air circulation and increased microclimate humidity, and seasons with frequent rainfalls, provide a significantly greater risk for PM development than their drier counterparts. Thick canopies and frequent rainfall are also associated with limited sunlight exposure, which greatly increases the risk of disease development in its own right. Collectively, these appear to be important environmental variables that distinguish between easy and challenging PM years (see below). (iv) Berries are extremely susceptible to infections initiated between the immediate prebloom period and fruit set, then become highly resistant to immune about 2 weeks (Concord) to 4 weeks (V. vinifera) later. This is when you use the good stuff and don't even think about cutting corners in terms of spray frequency and application technique. Your annual reminder. (v) Failure to control even inconspicuous PM infections on the berries can increase the severity of Botrytis at harvest, and can promote the growth of wine-spoilage microorganisms (such as Brettanomyces) on the fruit. Another annual reminder. Providing excellent PM control on susceptible wine grapes from pre-bloom right through bunch closing does not guarantee control of bunch rots and spoilage beasties, but it s a relatively easy way to eliminate one avenue for getting them started. (vi) Powdery mildew is a unique disease in that the causal fungus lives almost entirely on the surface of infected tissues, sending little sinkers (haustoria) just one cell deep to feed. This 2

3 makes it subject to control by topical treatment with any number of alternative spray materials (oils, bicarbonate and monopotassium phosphate salts, hydrogen peroxide, plant extracts, etc.), all of which have little to no effect on other disease-causing fungi that do their dirty work down inside the infected tissues. Recall that there are two primary limitations to the aforementioned group of products, which need to be considered if you want to use them effectively: (a) they work by physical contact with the fungus, so can only be as effective as the spray coverage that you provide; and (b) they work primarily in a post-infection/curative mode by killing the fungus right after they hit it, with only modest (JMS Stylet Oil) to zero (potassium salts) residual activity against any spores that land on the vine after these materials have been applied. This means that they need fairly frequent re-application, or should be tank-mixed with something that provides good protective (forward) activity in order to lengthen effective spray intervals. Once-new research I: Effect of sunlight exposure Everybody knows that PM is most severe in shaded regions of the vineyard (canopy centers, near trees, etc.). Here s another recap of the work of former graduate student, Craig Austin (now gainfully employed and paying taxes, bless him), who showed definitively just how profound this influence can be (and why): One of Craig s first experiments was conducted in a Chardonnay vineyard near the Finger Lakes village of Dresden, NY where a small portion of the easternmost row was bordered by a group of 50-foot tall pine trees. In previous years, we had seen PM completely destroy the clusters on the three panels of vines immediately next to the trees, despite a spray program that controlled the disease adequately on all other vines in the block. These panels were shaded by the trees during the morning and it wasn t until the sun crested over the treetops just before noon each day that the vines received their first direct exposure to sunlight. So, we initiated a trial in which Craig inoculated leaves on either (a) the outer edge (exposed) or (b) inner (shaded by the vine s own canopy) portions of individual vines, which were located either (i) immediately next to or (ii) 200 feet away from these trees, thereby providing a total of four levels of natural shade. The resulting disease severity increased substantially with each increasing level of shade, becoming 8 to 40 times more severe on the most heavily shaded leaves (interior of vines next to the trees) compared to the unshaded leaves on the exterior of vines away from the trees (Fig. 1). Figure 1. Percent area diseased on Chardonnay leaves receiving (i) full solar radiation, on the outer canopy edge of vines away from trees (No Shade); (ii) morning shade from an adjacent grouping of trees, but which were fully exposed to the sun for the rest of the day--i.e., leaves on the outer canopy edge of vines next to the trees (Trees); (iii) shade provided throughout the day by the vine canopy itself, i.e., leaves located within the center of the canopy of vines away from the trees (Canopy); or (iv) shade provided by both the trees and the vine itself, i.e., leaves located within the center of the canopy of vines next to the trees (Tree & Canopy). 3

4 Although shading could potentially change air temperature or relative humidity within the vine canopy, our measurements did not show this. However, they did show that UV radiation levels and leaf temperatures were dramatically different among the different treatments. Within the shaded regions, UV levels were (as one would expect) a mere fraction of those in the sun, and temperatures of leaves in the sun were as much as 10 to occasionally even 30 F higher than those of leaves in the shade. As we later found out, both elevated leaf temperature and UV radiation are responsible for the inhibitory effects of sunlight on PM development. Sunlight characteristics influencing powdery mildew development. As noted above, direct sunlight heats up exposed leaf surfaces, as it does anything else it hits--as we all know from the difference between standing in the sun or taking two steps away into the shade. On warm days, this additional heat can suppress or even kill PM colonies on sun-exposed leaves and berries. Recall that powdery mildew grows best at temperatures near 80 F, but stops growing at temperatures above 90 F and will start to die at temperatures above 95 F, depending on how hot it is and for how long. On a hypothetical spring or summer day in the 80's, temperatures of shaded leaves and clusters will remain near that of the air which is at or near the optimum for PM development. However, nearby vines or portions thereof that are fully exposed to sunlight often have temperatures elevated to a point where the PM fungus will stop growing or even start to die. UV radiation from the sun can damage the cellular structure of virtually all forms of life. However, as a disease powdery mildew is uniquely vulnerable to such damage: as noted previously, the PM fungus lives primarily on the outside of infected tissues, whereas nearly all other pathogens live and grow within infected organs, where they are protected from UV radiation. On top of that, the PM fungus is white--it has no pigment ( suntan ) to protect against this radiation. Lab experiments confirmed that doses of UV typically measured during a summer afternoon in the Finger Lakes region (hardly a world beater when it comes to sunshine intensity!) are deleterious to pathogen development and that this effect is even greater once leaf temperatures get in to 80 s and beyond. And we ve just discussed the increase in leaf temperature that occurs when they are exposed to the sun. Surface temperature and UV: Field experiments. In order to quantify the effects of each of these two specific sunlight components, Craig suspended a Plexiglas "roof" over Chancellor and Chardonnay vines in Geneva, NY and Chardonnay vines in a vineyard at Washington State University's Irrigated Agriculture Research and Extension Center in Prosser, WA (grateful acknowledgement to Dr. Gary Grove and staff for their collaboration with this part of the study). Plexiglas blocks UV radiation but permits passage of the sunlight wavelengths that elevate leaf temperature. At the Chancellor vineyard in Geneva, we also suspended shade cloth over other vines to block 80% of the available sunlight, thereby shielding them not only from most UV radiation but also from most rays of the sun that cause heating. Clusters were inoculated with PM spores at 75% capfall. As shown in Figure 2, we found that removing UV radiation while allowing exposed tissues to heat (Plexiglas filter) increased disease severity on fruit by anywhere from 50% to fivefold, for both varieties and locations. The Chancellor shade cloth treatment, which eliminated both UV radiation and the increase in temperature caused by sun exposure, further increased disease severity in one of the two experiments. 4

5 Figure 2. Percent cluster disease severity on cv. Chancellor and cv. Chardonnay vines receiving: (i) full solar radiation (Exposed); (ii) sunlight from which 95% of the UV radiation had been filtered (UV Filter); or (iii) sunlight reduced to 20% of ambient via neutral density shade cloth (Shade Cloth). Vineyards were located in Geneva, NY (Finger Lakes) or Prosser, WA (Yakima). Manipulating sunlight exposure to manage PM. Given that UV radiation and sun exposure reduce PM, how can we use this information to better manage the disease? We examined this question in a young Chardonnay vineyard in Geneva, NY by comparing two training systems, Vertical Shoot Positioning (VSP) and Umbrella-Kniffen (UK), and removing basal leaves around clusters to provide different levels of light exposure in the fruiting zone. UK provided more shoots per linear foot of row than VSP, hence more potential for canopy shading in the fruit zone. Within each training system, we removed basal leaves at one of two timings: 2 weeks postbloom (fruit set) or 5 weeks post-bloom. We inoculated clusters with PM spores at bloom, then rated disease severity in each treatment late in the summer. We found that both factors affected PM severity (Figure 3). First, powdery mildew severity was lower in the VSP than in the UK training system, regardless of leaf pulling treatment. Second, leaf removal at fruit set significantly reduced the amount of disease in both training systems, but leaf removal 5 weeks after bloom had no effect. The benefits of the early (versus late) leaf removal once again illustrates the critical nature of those first few weeks following the start of bloom--this is when you want to hit the fungus not only with your best spray program but also with the cultural control tools you have available. It s when you do or don t get control of the disease on berries. Bottom line: simply by utilizing a VSP training system and basal leaf removal at fruit set, we were able to reduce fruit disease severity by more than one-third relative to UK-trained vines with no leaf removal. It should be noted that in 2009, a summer during which it sometimes seemed that there was no direct sunlight reaching the state of NY, we did not see the same effect of training system in this vineyard but did see the same effect of early leaf pulling. 5

6 Figure 3. Powdery mildew severity on Chardonnay clusters subjected to five different leaf-removal treatments in each of two vine-training systems. Leaf-removal code: First letter is leaf removal severity, H = heavy, L = light (either two leaves or one leaf above and below each cluster, respectively); Second letter is leaf removal timing, E = early, L = late (2 and 5 wk post-bloom, respectively). Each data bar represents the mean for 30 clusters per treatment. Figure 4. Effect of canopy density on deposition of sprays onto clusters of Chardonnay vines, provided by a conventional airblast sprayer in mid-july. Exposure of fruit to sunlight and pesticides. It's common sense that canopy management practices that increase sunlight penetration into the fruiting zone should also increase the penetration of sprays applied to control pests and diseases. With the assistance of Dr. Andrew Landers, we were able to quantify the effect that canopy density can have on spray coverage. Vines in our Chardonnay planting subjected to the above canopy manipulations were sprayed with a conventional air blast unit and deposition on clusters from each vine was assessed in the lab. As expected, we found a direct relationship between the quantity of spray deposited on each cluster and the sunlight exposure level (Figure 4), with well-exposed clusters receiving approximately twice the deposition as those with poor exposure. Subsequently, Andrew Landers, Nicole Landers, and yours truly expanded this part of the study into four commercial Finger Lakes vineyards plus another experimental block that represented a range of cultivars (Chardonnay, Vignoles, Cabernet Franc, GR-7, and Rosette) and which were subjected to various canopy management practices. Canopy density was determined for replicate test panels in each vineyard on the basis of Cluster Exposure Layer (CEL), the average number of objects (usually leaves) between clusters and the sprayer. The deposition of a dilute foodgrade dye solution, applied with a Berthoud airblast unit in early July (a critical time for controlling multiple diseases on clusters), was then determined in the lab by measuring the dye washed from a sample of clusters from each of these same test panels. Finally, the average deposition in each test panel was graphed as a function of its CEL value, yielding Figure 5 below. 6

7 Figure 5. Effect of canopy density (cluster exposure layers = CEL) on deposition of a spray tracer dye onto grape clusters in 5 Finger Lakes vineyards. Vines were treated in early July with a conventional airblast sprayer applying 50 gallons/acre. Although individual data points show the typical variability around the average line indicated in red, the relationship between spray deposition and canopy density is clear. For example, clusters separated from the sprayer by one layer of leaves (CEL = 1.0) received more than twice as much spray as those separated by two layers (CEL = 2.0). Obviously this has significant implications for the management of all diseases and arthropod pests against which you spray, not just PM. Summary. In all vineyards, in all seasons, for all experiments at all locations, increasing sunlight exposure on leaves or fruit reduced the severity of powdery mildew on those tissues, independent of spray coverage. And when improved spray coverage is factored in, the benefit of canopy management for PM control is not only compounded but extends to other diseases as well. However, a central concept associated with quality viticulture is balance". Zero sunlight exposure might lead to diseased berries, but absolute maximum exposure can lead to sunburned berries instead. Again, it's all about balance. Once-new research II: What s a bad PM year? Current Washington State University viticulturist Michelle Moyer examined some different aspects of powdery mildew biology while working as a Cornell graduate student in the lab of Drs. David Gadoury and Bob Seem a few years back. Michelle focused on trying to define just what makes a bad PM year while it is occurring, so that growers might take action to prevent damage rather than conduct a post-mortem after it's too late; or, conversely, avoid making more sprays that they need to. 7